I think it's well worth it for two days. Lots of stuff to do. You can visit the Hohensalzberg castle, the Moenchsberg mountain, and the historic city center (including many worthwhile churches such as the cathedral).

I don't know the answer to your other questions--I'm not into Christmas markets and I've also never done a one-way rental in Austria (from my location in Germany, it's just more convenient to drive with my own car).